#seu12 - 507 an introduction to fea via solid edge and femap - mark sherman
DESCRIPTION
This session will cover the basics of Finite Element Analysis with an emphasis on how to use FEA tools to effectively influence the design process and increase product quality and performance. Proper application of the tools provided in both Solid Edge and FEMAP will be discussed. This session should be useful to designers and engineers who want to more fully understand the structural, dynamic, and thermal performance of individual parts and complex systems.TRANSCRIPT
#SEU12
An Introduction to FEA via Solid
Edge and FEMAP
Mark Sherman
Siemens PLM Software
FEMAP
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Introduction
Mark Sherman
610-458-6502
BS Aerospace and Ocean Engineering –
Virginia Tech
Boeing Helicopter
GE Astro Space
Joined FEMAP (Enterprise Software
Products, Inc. – 1992 )
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FEMAP History
• Founded by George Rudy, 1985
• Mission – PC-based dedicated Pre- and
Post-Processor for Engineering Finite
Element Analysis
• Originally a Pre-Processor Only for
MSC/PAL and MSC/NASTRAN on 64k RAM
IBM PCDOS to Windows Transition 1990-
1992
• Rapid Growth from 1992 on
• Significant strength in Aerospace
accounts
• Laboratory Module of the ISS (Boeing)
• OEM Partnership – MSC/Nastran for
Windows
• Tens of thousands of licenses worldwide
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FEMAP
Continuous development with the
same core team! Since 1985 there have been more than 30 releases of FEMAP with
only one major architecture change (DOS to Windows)
FEMAP Development Team is all engineers turned programmers –
FEA By Engineers for Engineers
Product development has been driven by FEA Analyst input
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Why use Finite Element Analysis
Understand the behavior of engineered parts and assemblies
Structural Behavior
Static – Stress, Deflection, Load Distribution - Linear
Dynamic
Natural Frequency – Normal Modes
Frequency Response (Sinusoidal) – Accel., Disp., Stresses
Transient Response (General Time-Varying Loads) Accel., Disp. Stresses
Non-Linear
Contact
Geometric Nonlinearity
Material Nonlinearity
• Thermal
Steady-State
Transient
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Why use Finite Element Analysis
Quality
Material Cost
Weight
Reduce requirement for physical testing
Maximize these benefits - Important to integrate FEA into the Design Process
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Customer Success Stories - Duracast
Customer was driven to seek a CAE solution
Rising cost of materials – need to optimize their current designs to save material use and cost (use petroleum based products)
To reduce the level of physical prototyping to save time and cost
Maintain integrity of product – advanced solutions required including nonlinear analysis
A single seat of FEMAP with NX/Nastran saves this company hundreds of thousands of dollars per year in material cost
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Space Exploration Technologies
Launch Vehicle Developer
Challenge: Develop rockets that
reduce the cost of space access by a
factor of 10
Approach: Create virtual mockups of
entire rockets using Femap and NX
software
Results: More effective collaboration
between design groups, and a 50%
productivity improvement helped
successfully launch two Falcon 1
rockets
"On the analysis front Femap and
NX Nastran were the clear
winners, not only due to wide
industry acceptance but also from
an ease of use and support
standpoint.”
Chris Thompson, vice president of
Development Operations
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A Brief History of FEA and FEM
The concept of a “Finite Element” was introduced by Prof. R.W. Clough of UC
Berkeley in 1960 at an ASCE Conference.
NASTRAN (NASA STRuctural ANalysis) was developed for NASA by a consortium
of several companies for the analysis of the Saturn V rocket.
Siemens PLM Software acquired MSC.Nastran source code in 2003 and has
greatly improved the performance and capabilities of
NX Nastran through the latest release of NX Nastran 8.1
Finite Element Modelers(Pre/Post Processors), the tools used to generate Finite
Element meshes and view results, were first commercialized in the 1970s.
Siemens PLM Software began the first commercial offering of FEM software with
the introduction of SDRC SuperTab in the 1970’s.
Siemens continues to support the analysis community with Femap and NX CAE
pre/post-processors.
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The Solution
Consider a single degree of freedom system – a simple spring:
Apply the following conditions to generate a system of simultaneous equations
where displacements are the unknowns:
Equilibrium of forces and moments
Strain- displacement relations
Stress-strain relations
K: spring stiffness P: applied load
u: displacement
K u = P (static analysis)
?
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Solution for Multiple DOFs
Any real structure can be modeled as a collection of elements connected
at nodes
With many elements and nodal dof’s, a matrix approach to the solution is
adopted
All element matrices are assembled into a global stiffness matrix
Kgg =
k11 k12
k21 k22 ka =
Element stiffness matrix ka kb
1 2 3
ka11 ka12
ka21 ka22 + kb22 kb23
kb32 kb33
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Modeling of Real Structures
• The behavior of the real structure is obtained by considering the collective behavior of the discrete elements.
• The user is responsible for the subdivision or discretization of real-world structures.
• Element choice has significant influence on the behavior
• A graphic preprocessor such as FEMAP/SE Simulation is the key tool for generating a model that accurately simulates real world structures
Kgg =
ka -ka
-ka ka + kb -kb
-kb kb
• Contributions from all other elements
n x n
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Why use Finite Element Analysis
Understand the behavior of engineered parts and assemblies
Structural Behavior
Static – Stress, Deflection, Load Distribution - Linear
Dynamic
Natural Frequency – Normal Modes
Frequency Response (Sinusoidal)
Transient Response (General Time-Varying Loads)
Non-Linear
Contact
Geometric Nonlinearity
Material Nonlinearity
• Thermal
Steady-State
Transient
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Linear Static Analysis
90%+ of all FEA projects
100% Linear – if you double the loads, you get double the response
Material stays in the elastic range – return to original shape
Small Deformation Maximum Displacement much smaller than characteristic dimensions of the part being studied, i.e. displacement much less than the thickness of the part
Loads are applied slow and gradually, i.e. not Dynamic or Shock Loading
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FEA Building Blocks
Numerous Element Types
This makes it possible to accurately model
the real-world performance of your
engineered structure
Element selection helps balance model size
vs.
Solution accuracy
Hardware resources
Solution time
Results interpretation time
Another consideration for solids is time to
mesh the model if a hex mesh is desired
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Choose elements to represent the real structure behavior
Rod Element: Axial and Torsion Stiffness only (pin connected truss)
Beam : Classic Euler-Bernoulli Beam depending on property options used
Shear effects, shear center offset, etc can be included, user must understand the
defaults and options to ensure proper behavior is included
Plate/Shell : Started with Kirchhoff and Mindlin theories but now many different
“tweaks” and modifications included to improve accuracy.
When elements of different types connect, user must be aware of potential
compatibility problems and use special modeling techniques as needed.
Examples: beams connecting normal to plates, plates connecting to solids
Element Basics
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Finite Elements – 0D
Scalar Elements
Springs
Node to node axial or torsional
Dampers
Node to node axial or torsional
Mass
Point masses can be used to represent additional mass and inertia in the structure
that is non-structural or where modeling detail is not required
Rigid Elements
Can be used to represent rigid connections within the model
17
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Finite Elements – 1D
Line Elements
Rod
Uniaxial tension compression and
torsion – no bending or shear loads
Used to model pin-ended truss
structures
Bar/Beam
A regular beam that carries axial,
torsion, bending and shear loads
Very versatile – offsets and tapers can
be included
18
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Finite Elements – 2D
Plate and Shell Elements
Used to represent “thin” structures like
sheet metal structures.
Additional features for shear-only,
membrane-only, composite laminate
etc.
Quad4 / Tria3
Isoparametric 3/4 noded
triangular/quadrilateral
Quad8 / Tria6
Higher order isoparametric 6/8 noded
triangular/quadrilateral
19
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Finite Elements – 3D
Solid Elements
Used to fill and model solid volumes
Hexa
Regular hexahedral elements
Penta
Pentrahedral used to mesh transitions
Tetra
Tetrahedral elements can be generated fully
automatically on solids
20
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Linear Static Analysis
What can you expect to learn from a linear static Finite Element Analysis
Displacements
Load Paths
Stress*
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Linear Static Analysis – Modeling Guidelines
Use the
Displacements
Load Paths
Stress*
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Linear Analysis is small displacement, small angle theory
Must use nonlinear analysis if the displacement changes the stiffness or loads
Pressure loads on flat surfaces, have no membrane component unless nonlinear large
displacement solution performed.(load carried by bending stiffness only)
Linear contact is a misnomer, contact condition is iterative solution, but no other nonlinear
effects are considered.
Mesh density required is a function of the desired answers
Must have enough nodes so model can deform smoothly like the real structure.
In general, accurate stresses require more elements than accurate
displacements.
Goal is for a small stress gradient across any individual element
Normal modes should always be run before any dynamic solution
Confirm model behavior, stiffness and mass properties are correct
Important Guidelines
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Finite Element Analysis is an Approximate Solution to a Complicated
Problem :
Therefore, Sound Engineering Judgment is Required
Our Answers are only as good as the assumptions we make
Common sources for analysis uncertainty:
Numerical round off (usually small)
FEM : mesh density, element formulation, element connections
Boundary conditions
Loads and environments seen by the structure
Important Guidelines
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Linear Statics - Stresses
To accurately recover stresses in shell and solid elements, the mesh must
be very dense in areas of high stress gradients
Stress Changing Too
Fast Across One Element
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Stresses from the Web
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Linear Statics - Stresses
To accurately recover stresses in shell and solid elements, the mesh must
be very dense in areas of high stress gradients
Stress Changing Less Across
an Element – More Accurate
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Linear Statics - Stresses
Keeping Model Size “Reasonable”
Increase the Mesh Density where you need it, decrease it where you don’t
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Linear Statics - Stresses
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Guidelines for Good Stress Interpretation -
Singularities
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Guidelines for Linear Static Analysis - Stresses
• Remember the limitations of “Linear” analysis
• Increase Mesh Density in High Stress Regions
• Ignore Stress Answers at Singularities
• Zero Radius Fillets
• Inside Corners
• Loaded and Constrained Nodes
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When you need FEMAP
Monaco Coach
Solid Mesh Impractical
Shells alone, not enough
Beam – Lumped Mass required
Transient Dynamic Analysis a
requirement
Courtesy of Predictive Engineering
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Real FEA - Examples
Bechtel River Project
Solid Mesh Impractical
Shells alone, not enough
Beam – Rigid, and Lumped Mass required
Transient Dynamic Analysis a requirement
Post-processing Data Processing Required
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Real FEA - Examples
Boeing - International Space Station
Laboratory Module
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Real FEA - Examples
Why can’t we just tetra-mesh this?
Just this little section is 565,405 Nodes – 275,558 Elements and there’s
only one element through the thickness
Full Model would be billions
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SE Simulation FEMAP
Model Size Limitations – when one has to idealize a structure beyond
the Solid, Shell and Beam Elements available in SE Simulation
Modeling Limitations
• Composite Laminates
• Nonlinear Geometry - Large
Displacements
• Nonlinear Materials - outside
the elastic range, or non-elastic
materials (rubber).
• Time or Temperature Dependent Loading
• Specialty Elements like CBUSH where you can have displacement
dependent stiffness and damping
• CWELD - CFAST
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SE Simulation Continues to Dive Deeper into FEA
Functionality
Without Pre-Load/Contact – First Mode 1.472608 Hz
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SE Simulation Continues to Dive Deeper into FEA
Functionality
With Pre-Load/Contact – First Mode 7.586033 Hz
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SE Simulation Continues to Dive Deeper into FEA
Functionality
First Mode 415% Higher!!!
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Why use Finite Element Analysis
Understand the behavior of engineered parts and assemblies
Structural Behavior
Static – Stress, Deflection, Load Distribution – Linear
Linear Contact
Dynamic
Natural Frequency – Normal Modes
Frequency Response (Sinusoidal)
Transient Response (General Time-Varying Loads)
Non-Linear
Contact
Geometric Nonlinearity
Material Nonlinearity
• Thermal
Steady-State
Transient
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Advanced Dynamics Examples
Sample Model
Tubular Structure
Supports Offset Payload/Mass
Subject to Lateral +/- X Forces
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Advanced Dynamics Examples
Idealized Model – beam elements for tubes and lugs, shell mesh at base
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Advanced Dynamics Examples
Always do a modal run first and make sure everything makes sense
Evaluate basic modes/natural frequencies
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Advanced Dynamics Examples
Frequency Response – Check Response at the end of the support arm to
frequencies between 0 and 30 Hz.
Request responses
between 0.0 and 30.0,
every 0.2 Hz
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Advanced Dynamics Example
Frequency Response – Limit Output, in this sweep, we are asking for 150
sets of Output, recover responses where they matter most, in this case, out
at the end of the arm.
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Advanced Dynamics Example
Use Stick and Plate model for preliminary design/sanity check
Update design as necessary
Create detailed Shell or Solid model of final design, run Frequency
Response at actual operating frequencies for final validation
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Advanced Dynamics Example
Stick and Plate Model – 2104
Nodes
All Shell Model – 18,115
Nodes
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Frequency Response Results
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Overview - Discussion
To get good results, accurately model your structure
• Material Properties, linear, non-linear
• Linear or nonlinear overall behavior
• Boundary Conditions
• Loads
• Structure – Does your mesh accurately reflect the structure
• Most Critical Stress condition may not be covered by linear static analysis
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Contact Information
Mark A. Sherman
610-458-6502
#SEU12
Thank You! Questions?